The present invention relates to an apparatus and a method for controlling air-fuel ratio of engine, and, more particularly, to an apparatus and a method for controlling air-fuel ratio that are suitable for use in an engine equipped with a vapor purge system which purges (discharges) vapor (fuel vapor) produced in a fuel tank into an engine intake system and processes the vapor.
In general, a vehicle equipped with a volatile liquid fuel tank employs a vapor purge system mentioned above. A typical charcoal canister type purge system temporarily collects vapor, produced in a fuel tank, in a canister. The canister incorporates an adsorbent, such as activated charcoal, and is constructed in such a way as to be able to temporarily adsorb the vapor in the adsorbent and desorb the vapor stored in the adsorbent as the canister is placed under a pressure lower than the atmospheric pressure. The vapor caught in the canister is purged, as needed, from the canister to the engine intake system through a purge line and mixed into the air fed into the engine. As the vapor is burned, together with the fuel injected from an injector, in a fuel chamber of the engine, the vapor produced in the fuel tank is processed.
There is known an air-fuel ratio control apparatus for an engine, which controls the air-fuel ratio of a flammable mixture of air and fuel supplied to a fuel chamber of the engine or the ratio of the amount of injected fuel (the amount of fuel supplied from a fuel feeding apparatus) to the amount of the intake air. Such a control apparatus performs feedback correction of the amount of injected fuel supply from an injector in such a way that the real air-fuel ratio detected by a sensor coincides with a target air-fuel ratio.
In an engine equipped with the purge system, however, a purge gas containing the aforementioned vapor is added to the original mixture to be supplied to the fuel chamber. Therefore, to adapt control that demands a strict control of the amount of supplied fuel to be burned in the fuel chamber, such as air-fuel ratio control, to an engine equipped with the purge system, it is necessary to adjust the amount of fuel supply taking the influence of the purge gas into consideration on such control.
In this respect, air-fuel ratio control taking the influence of a purge gas into consideration has conventionally been achieved as follows. For a correction value of the amount of fuel supply that is associated with the feedback of the air-fuel ratio (air-fuel ratio feedback correction value), the density (vapor density) of a fuel component in the purge gas is estimated from changes in a value detected when the flow rate of the purge gas changes. Thereafter, the flow rate of vapor to be supplied to the engine through purging is acquired from the vapor of the estimated fuel component and the flow rate of the purge gas, and the amount of fuel injected from an injector is corrected to become smaller accordingly. Every time the drive condition of the engine satisfies a predetermined condition, the vapor density is likewise obtained and the control is adapted by correcting the estimated value.
The air-fuel ratio control in such a mode sufficiently and effectively works when the vapor density is constant regardless of the purge flow rate and a change in the density of a vapor component in the purge gas is sufficiently gentle. That is, air-fuel ratio control is adapted on the premise that the purge flow rate to an engine intake passage and the flow rate of vapor contained in the passage have a linear relationship.
When a large amount of vapor is produced, such as at the time of feeding fuel, excess vapor may be adsorbed by the adsorbent temporarily, thus deteriorating the adsorbent. To cope with this problem, therefore, a purge system designed to have adsorbent-unfilled space in a canister and suppress the degradation of the adsorbent by using the layer of air (canister air layer) in that space as a buffer band has been proposed as disclosed in, for example, Japanese Unexamined Patent Publication No. Hei 9-184444.
In such a purge system, depending on the circumstance, part of vapor generated in the fuel tank may pass through the canister air layer and is directly purged into the intake passage of the engine without being caught by the adsorbent.
On the assumption that vapor flows into the engine, the air-fuel ratio control apparatus for an engine described in this publication adapts control in anticipation of the influence of the purge gas in the following two modes:
(a) a mode in which vapor is directly purged into the intake passage from the fuel tank without being adsorbed by the adsorbent, and
(b) a mode in which vapor is temporarily adsorbed by the adsorbent, then desorbed therefrom and purged into the intake passage.
In the following description, purging in the former mode (a) is called xe2x80x9cflow-from-tank purgingxe2x80x9d and purging in the latter mode (b) is called xe2x80x9cdesorption-from-adsorbent purgingxe2x80x9d. The behavior of vapor during purging naturally differs between those xe2x80x9cflow-from-tank purgingxe2x80x9d and xe2x80x9cdesorption-from-adsorbent purgingxe2x80x9d. As a result, the linear relationship between the purge flow rate and the vapor flow rate, which is one of the premises for the control, does not stand always. Even with the vapor flow rate to the intake passage being the same, for example, the behavior of vapor during purging becomes quite different between a case where there is vapor flowing from the fuel tank and a case where there is not.
The air-fuel ratio control apparatus for an engine described in the above-mentioned publication separately acquires a vapor flow rate Fvptnk for the xe2x80x9cflow-from-tank purgingxe2x80x9d to the intake passage and a vapor flow rate Fvpcan for the xe2x80x9cdesorption-from-adsorbent purgingxe2x80x9d to the intake passage. The two vapor flow rates are computed in separate calculation modes and an estimated value Fvpall of the total flow rate of vapor to be purged into the engine intake system (the total vapor flow rate) is acquired from the computed vapor flow rates.
Specifically, the vapor flow rates Fvptnk and Fvpcan are calculated from the following equations, the total (Fvptnk+Fvpcan) is estimated as the total vapor flow rate Fvpall and the amount of fuel injection from an injector is corrected based on the estimated value.
 less than  less than Reference Formulae greater than  greater than 
Fvptnk←rvptnk/(Qxc2x7Fpgall) 
Fvpcan←rvpcanxc2x7Fpgall 
Fvpall←Fvptnk+Fvpcan 
where xe2x80x9cQxe2x80x9d indicates the amount of intake air, xe2x80x9crvptnkxe2x80x9d indicates the vapor density in flow-from-tank purging (the ratio of vapor content in the purge gas) and xe2x80x9crvpcanxe2x80x9d indicates the vapor density in desorption-from-adsorbent purging.
In other words, the air-fuel ratio control apparatus for an engine described in the publication separately computes the vapor flow rate Fvptnk in flow-from-tank purging and the vapor flow rate Fvpcan in desorption-from-adsorbent purging and computes the total vapor flow rate Fvpall as the sum of the two vapor flow rates.
Estimation of the vapor flow rate in the above-described manner can allow the vapor flow rate to be estimated in accordance with a variation in vapor density condition that is caused by whether vapor flows into the canister from the fuel tank or not. Therefore, a certain improvement on the precision of air-fuel ratio control or the like can be expected.
However, it is confirmed through tests or the like conducted by the present inventors that the vapor behavior in an actual purge system is far more complex than the one assumed at the time of setting a logic of estimating the vapor flow rate in the control apparatus. Even in case where the logic of calculating the vapor flow rate in the mode described in the publication, therefore, the calculation accuracy cannot be increased sufficiently and there is naturally a limit to the suppression of the influence of purging on the air-fuel ratio control or the like.
Accordingly, it is an object of the present invention to provide an apparatus and a method for controlling air-fuel ratio of an engine equipped with a vapor purge system which purges and processes vapor generated in a fuel tank and that adequately restrains the influence of purging on the air-fuel ratio control or the like by estimating the purging-originated vapor flow rate to the engine more accurately.
To achieve the object, the present invention provides an air-fuel ratio control apparatus for controlling the air-fuel ratio of air-fuel mixture drawn into a combustion chamber of an engine. A canister is connected to an intake system of the engine through a purge line. The canister includes an adsorbent, an air layer located between the adsorbent and the purge line, and an air hole for introducing air into the canister. The adsorbent adsorbs fuel vapor generated in a fuel tank and permits adsorbed fuel vapor to be desorbed. Air introduced into the canister through the air hole flows to the purge line through the adsorbent. Gas containing fuel vapor is purged to the intake system from the canister through the purge line. The apparatus includes a computer, which performs feedback correction of the amount of fuel supplied to the combustion chamber such that the air-fuel ratio of the air-fuel mixture seeks a target air-fuel ratio. By using a physical model related to the fuel vapor behaviors, the computer estimates a total vapor flow rate, which represents the flow rate of fuel vapor in gas purged to the intake system, according to a total purge flow rate representing the total flow rate of the purged gas. The physical model is based on a physical status quantity representing the fuel vapor stored state of the air layer, a physical status quantity representing the fuel vapor stored state of the adsorbent, and a physical status quantity representing the vapor generating state in the fuel tank. According to the estimated total vapor flow rate, the computer corrects the fuel supply amount, which is subjected to the feedback correction.
The vapor behavior in the vapor purge system can be explained a physical model based on three physical status quantities (see FIGS. 13 and 46), or the vapor stored state of the air layer in the canister, the vapor stored state of the adsorbent in the canister, and the vapor generating state in the fuel tank. The vapor behavior in the purge system changes incessantly in accordance with the purging state and the fuel vapor generating state in the fuel tank. Since being based on the listed physical status quantities, the above physical model accurately estimates the flow rate of fuel vapor purged to the intake system through the purge line (the total vapor flow rate Fvpall) in accordance with changes of the vapor behavior. Therefore, regardless of changes in the vapor behavior in the purge system, the flow rate of fuel vapor purged to the intake system through the purge line is accurately predicted. This permits the air-fuel ratio to be accurately controlled during purging.
The present invention also provides a method for controlling the air-fuel ratio of air-fuel mixture drawn into a combustion chamber of an engine. A canister is connected to an intake system of the engine through a purge line. The canister includes an adsorbent, an air layer located between the adsorbent and the purge line, and an air hole for introducing air into the canister. The adsorbent adsorbs fuel vapor generated in a fuel tank and permits adsorbed fuel vapor to be desorbed. Air introduced into the canister through the air hole flows to the purge line through the adsorbent. Gas containing fuel vapor is purged to the intake system from the canister through the purge line. The method includes: performing feedback correction of the amount of fuel supplied to the combustion chamber such that the air-fuel ratio of the air-fuel mixture seeks a target air-fuel ratio; obtaining a physical status quantity representing the vapor stored state of the air layer; obtaining a physical status quantity representing the fuel vapor stored state of the adsorbent; obtaining a physical status quantity representing the vapor generating state in the fuel tank; estimating a total vapor flow rate, which represents the flow rate of fuel vapor in gas purged to the intake system, according to a total purge flow rate representing the total flow rate of the purged gas by using a physical model related to the fuel vapor behaviors, wherein the physical model is based on the obtained physical status quantities; and correcting the fuel supply amount, which is subjected to the feedback correction, according to the estimated total vapor flow rate.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.